Composition for blackening copper

ABSTRACT

Novel blackening compositions comprising a peroxydiphosphate compound and an alkali metal hydroxide are utilized to produce on copper surfaces a soft, velvet-black coating which can be mechanically rubbed to form a smooth, black finish. The coating process is carried out by immersing the copper article in the aqueous blackening bath at a temperature of about 150* to about 210* F. for about 3 to about 30 minutes.

ates

Grunwald et al.

[54] COMPOSHTKON FOR BLACKENHNG COPPER [72] Inventors: John J. Grunwald,150 Roydon Road,

New Haven, Conn. 06511; Edmund E. Hornet, 105 Crestview Drive,Watertown, Conn. 06795 22 Filed: May 15,1970

211 Appl.No.: 48,766

52 use]. ..l48/6.l5R,252/416 [51] Int. Cl. ..C23f 7/12 [58] Field ofSearch ..148/6.l5 R, 6.14; 252/416 [56] References Cited UNITED STATESPATENTS 2,481,854 9/1949 MacMahon 148/6. 1 4

[151 3,7, [451 Apr.1&197

2,817,610 12/1957 Newell ..148/6.14 3,279,957 10/1966 Fink ..148/6.14

Primary Examiner-Ralph S. Kendall Attorney-Steward & Steward, Merrill F.Steward, Donald T. Steward and Walter D. Hunter [57] ABSTRACT 19 Claims,N0 Drawings COMPOSITION FOR BLACKENING COPPER This invention relates tocompositions useful for producing colored coatings. More particularlythis invention relates to compositions comprising a peroxydiphosphatecompound and sodium or potassium hydroxide and to a process in whichsuch novel compositions are utilized in blackening copper metal, copperplate or copper-rich alloys, such as brass and bronze. The novelblackening compositions of this invention produce a soft, velvet-blackfinish on the copper surface which may be mechanically rubbed to form asmooth, black finish.

The blackening of copper is an important metal finishing operation whichis carried out for a variety of purposes such as for aesthetic appeal,to reduce light reflection in optical devices as exemplified by rangefinders, periscopes, binoculars, etc., having copper or copper alloyelements which unless coated in a suitable manner may generateundesirable secondary reflections, shadow images, etc. Copper and copperalloy surfaces are also sometimes blackened to form a suitable undercoatfor subsequent organic coating and to improve the adhesion of sheetcopper employed in preparing copper-clad,

fiber-glass, epoxy laminates.

The blackening of copper surfaces with aqueous alkaline solutions ofeither ammonium persulfate, potassium chlorite, or sodium sulfide hasbeen disclosed in a number of processes known in the art. While each ofthese oxidizing baths will blacken copper, they all suffer from one ormore serious disadvantages. For example, alkaline solutions of sodium,potassium or ammonium persulfates are very unsatisfactory because thepersulfates themselves are unstable and the life of the solution isrelatively short, i.e., usually less than about eight hours. Blackeningbaths employing sulfides such as sodium or ammonium sulfide or otherwater-soluble sulfides have not been found to be satisfactory blackeningagents since the coating formed has relatively poor adhesion. Further,such sulfide films are unstable when exposed to outside atmospheres andthus must be protected by an organic coating, such as a lacquer or awax, which must be renewed from time to time. The use of potassiumchlorite or sodium chlorite in preparing blackening compositionspresents a manufacturing hazard in that these materials, that is thechlorites themselves, must be stored in restricted areas since suchcompositions are explosive upon percussion in the presence of organicmatter and when brought into contact with organic material such asfabric, wood, paper, etc., there is a definite fire hazard. Also, thechlorites when exposed to acids are known to produce noxious fumes ofchlorine dioxide in dangerous amounts.

The novel compositions of this invention comprise a mixture of aperoxydiphosphate compound, such as potassium peroxydiphosphate (i.e.,K.,P O together with sodium or potassium hydroxide. Usefulperoxydiphosphate compounds include ammonium peroxydiphosphate; thealkali metal peroxydiphosphates, such as sodium, potassium and lithiumperoxydiphosphates; the alkaline earth metal peroxydiphosphates such asthe calcium and magnesium salts, and mixtures thereof. Preferably, thecompositions are formulated as pulverulent mixtures. Such compositions,when dissolved in a suitable amount of water, fonn solutions which areparticularly useful for coloring copper or copper alloy surfaces a deepblack.

in contrast to the instability of the alkaline baths set forth in theart, the novel bath compositions of this invention are stable over longperiods of time and the coatings formed on copper and copper alloys givelong lasting and excellent protection against atmospheric and otherinfluences.

Although an aqueous solution of a peroxydiphosphate compound, such aspotassium peroxydiphosphate, will not blacken copper even when theconcentration of the phosphate is as much as 120 grams per liter,surprisingly, it has been found that the novel compositions of thisinvention which include an The novel dry compositions of this inventioncan be formulated, for example, by simply mixing powdered or flakedanhydrous sodium hydroxide or potassium hydroxide with the powderedperoxydiphosphate compound. Any suitable mixing device such as anordinary mixing drum employed for admixing a fine powder is suitable forpreparing the novel dry compositions of this invention. Since sodiumhydroxide and potassium hydroxide are hydroscopic materials, preferablythe mixing operation is carried out in a dry atmosphere and the mixed,prepared dry composition is also preferably kept out of contact withmoisture in order to avoid any tendency toward caking after the powderhas been formulated.

The novel dry blackening compositions of this invention preferably willcontain from about 7.7 to about 66.7 weight percent of theperoxydiphosphate compound with the balance being sodium hydroxide orpotassium hydroxide. In employing the dry blackening compositions toprepare the aqueous blackening baths or solutions a sufficient quantityof the dry composition is mixed with water to yield a blackening bathcontaining from about 5 to about grams per liter of theperoxydiphosphate compound and from about 60 to about 120 grams perliter of sodium or potassium hydroxide. The water employed in preparingthe blackening bath compositions may or may not be preheated before thedry composition is dissolved and the solution may be agitated during themixing operation, if desired.

The solutions utilized in the copper blackening process of thisinvention need not necessarily be prepared from a previously constitutedmixture of the peroxydiphosphate compound and an alkali metal hydroxidesince the individual ingredients, i.e., the peroxydiphosphate and thealkali metal hydroxide may be added separately, if desired, to aquantity of water in order to form the blackening bath solution. The useof the novel dry compositions of this invention in preparing theblackening bath or solution does, however, offer a number of distinctadvantages especially with regard'to handling and shipping.

Generally, the blackening process of this invention is operated with thebath temperature maintained between about to about 210 F. and preferablyfrom about to about 205 F. It has been found that the temperaturerequired for the blackening operation varies somewhat with theconcentration of the chemicals in the bath in that at higher chemicalconcentrations lower temperatures may be employed. In a like manner, thecoating developed, which may range from a smooth, dull black to a velvetblack, is influenced by the operating variables such as the time ofimmersion of the copper or copper alloy surface in the bath, thetemperature of the bath and the concentration of chemicals in the bathitself. The immersion time in the bath required to develop asatisfactory black coating on the surface of the copper or copper alloybeing treated, generally can be varied over a wide range and usuallywill be from about 3 to about 30 minutes or more depending upon theother operating conditions.

Prior to immersing the surface of the copper or copper alloy article inthe aqueous blackening bath of this invention, it has been found highlyadvantageous to first subject the copper article to a soaking action inany of the conventionally employed alkaline cleaning baths, followed byan acid dip. The aqueous alkaline cleaning bath can be, for example, anadmixture of trisodiurn phosphate, sodium carbonate and analkaline-stable surfactant to facilitate wetting of the surface or anyof the other numerous alkaline cleaning compositions well known in theart can be employed. In the second step of the cleaning operation theacid dip employed can be, for example, sulfuric acid, nitric acid,sulfuric acid-nitric acid bright dip, chromic acid bright dip, etc.Preferably, after the alkaline cleaning step and subsequent to the acidtreatment step, the article is thoroughly washed with cold water.

alkali metal hydroxide, such as potassium or sodium hydrox- 70 Thecopper blackening process of this invention comprises ide, will yield asolution which will blacken copper metal, copper plate or copper alloysand impart a velvet black-brown coating on berrylium copper. Further,mechanically rubbing of the black-brown coating will yield a smooth,black, adherent finish.

the following steps:

1. subjecting the surface of the copper or copper alloy article to theaction of an alkaline cleaning solution, 2. rinsing the article in coldwater, 3. immersing the article in an acid dip,

4. rinsing the article in cold water,

5. blackening the surface of the article thus cleaned by immersing it ina hot aqueous solution of the peroxidiphosphate compound and an alkalimetal hydroxide,

6. rinsing the blackened article in cold water followed by rinsing inhot water, and

7. drying the thusblackened article.

The drying of the blackened copper surface may be accomplished in avariety of ways well known in the art such as, for example, by passingthe articles on a conveyor through a tunnel drier maintained at atemperature of about 150 F.; or the blackened articles after beingrinsed with hot water may be allowed to dry at room temperature underambient conditions.

The coatings imparted to copper surfaces utilizing the process of thisinvention may vary from a smooth, dull black to a velvet black dependingon the operating conditions. The velvet black coatings developed may bereadily brushed to a smooth finish and while the velvet is readily wipedsmooth, it is very adherent.

The following examples illustrate various embodiments of this inventionbut are to be considered not limitative:

EXAMPLE I A blackening solution was prepared by mixing sufiicient finelydivided potassium peroxydiphosphate and sodium hydroxide with water togive a final solution containing about 120 grams per liter of potassiumperoxydiphosphate and 60 grams per liter of sodium hydroxide. The mixingtemperature was about 150 F.

A copper sheet was cleaned by first soaking it in an alkaline cleaningsolution, rinsing with cold water and then immersing it in an acid dipfollowed by a final cold water rinse. The clean copper sheet wasimmersed in the blackening solution previously prepared for 5 minutes ata temperature of about 195 F. On removal from the blackening bath, thecopper sheet possessed a uniform, adherent, velvet black coating on itssurfaces. After the sheet had been subjected to a cold water rinse,followed by a hot water rinse, it was then allowed to dry at roomtemperature. The coating was wiped smooth yielding a copper sheet havingan adherent smooth, dull, black finish.

EXAMPLES II-XVI TABLE 1 Example No. 111

Blackening Solution KPgOK NaOH(g./l.)... KOH (g./1/) Temperature (F.l180 Immersion Time (min.)..... 5 5

a'als Example No. Xll

Blackening Solution K4P2 fi!(g-/1-) NaOH (g./l.)... KOH (g.ll.).........Temperature CF.) Immersion Time (min.).....

as a

ass

Example No.

Blackening Solution (N HIM- 0,. tg./l.) N t z fl (S-/ NaOH (LL/1.)...KOH (g./l.)..... Temperature Immersion Time( In this example the bathwas agitated since some precipitation took place during the blackeningoperation.

Whatjs claimed is: g

1. The process of forming an adherent black coating on the surface of acopper or copper alloy article which comprises immersing the surface ina hot solution comprising water, a peroxydiphosphate compound selectedfrom the group consisting of ammonium peroxydiphosphate, an alkali metalperoxydiphosphate and an alkaline earth metal peroxydiphosphate andmixtures thereof, and an alkali metal hydroxide selected from the groupconsisting of sodium hydroxide and potassium hydroxide and mixturesthereof, said peroxydiphosphate being present in solution at aconcentration of at least 5 grams per liter and the ratio ofperoxydiphosphate to hydroxide being at least 1:12 on a dry weightbasis.

2. The process of claim 1 wherein the temperature of the solution rangesfrom about 150 to about 210 F.

3. The process of claim 1 wherein the concentration of the saidperoxydiphosphate compound is from about 5 to about grams per liter.

4. The process of claim 1 wherein the said peroxydisphosphate compoundistassium roxydiphos hate.

. The process of clatm wherein e concentra ion of the alkali metalhydroxide is from about 60 to about 120 grams per liter.

6. The process of claim 1 wherein the alkali metal hydroxide is sodiumhydroxide.

7. The process of claim 1 wherein the alkali metal hydroxide ispotassium hydroxide.

8. The process of claim 1 wherein the said article is immersed in thehot solution for about 3 to about 30 minutes.

9. The process of claim 1 wherein the said article is immersed for about3 to about 30 minutes in a solution comprising water, potassiumperoxydiphosphate and sodium hydroxide maintained at a temperature ofabout to about 210 F.

10. The process of claim 1 wherein the said article is immersed forabout 3 to about 30 minutes in a solution comprising water, potassiumperoxydiphosphate and potassium hydroxide maintained at a temperature ofabout 150 to about 210 F.

11. An aqueous solution for use in forming an adherent black coating onthe surface of a copper or copper alloy article comprising, in additionto water, a peroxydiphosphate compound selected from the groupconsisting of ammonium peroxydiphosphate, an alkali metalperoxydiphosphate and an alkaline earth metal peroxydiphosphate, andmixtures thereof, and an alkali metal hydroxide selected from the groupconsisting of sodium hydroxide and potassium hydroxide and mixturesthereof, said peroxydiphosphate being present in solution at aconcentration of at least 5 grams per liter and the ratio ofperoxydiphosphate to hydroxide being at least 1:12 on my ws shtb sis V-V.

12. The aqueous solution of claim 11 wherein the concentration of theperoxydiphosphate compound is from about 5 to about 120 grams per literand the concentration of the alkali metal ydt x a sab utfiQte zeut1.grams p l 13. The aqueous solution of claim 12 wherein the saidperoxydiphosphate compound is potassium peroxydiphosphate.

14. The aqueous solution of claim 11 wherein the alkali metal hydroxideis sodium hydroxide.

15. The aqueous solution of claim 11 wherein the alkali meta hyq q essisp t s u miuetq u L.

16. A dry composition useful in forming an adherent black coating on thesurface of a copper or copper alloy article comprising aperoxydiphosphate compound selected from the group consisting ofammonium peroxydiphosphate, an alkali metal peroxydiphosphate, and analkaline earth metal peroxydiphosphate, and mixtures thereof, and analkali metal hydroxide selected from the group consisting of sodiumhydroxide and potassium hydroxide, said peroxydiphosphate being presentin a weight ratio of from 1:12 to 2:1 relative to said alkali metalhydroxide.

17. The dry composition of claim 16 wherein the said peroxy h n e empefieu isrstassiim a r u phq p atc- 18. The composition of claim 16wherein the said perox- 19. The composition of claim 16 wherein the saidperoxydiphosphate compound is potassium peroxydiphosphate ydiphosphatecompound is potassium peroxydiphosphate present in an amount equal toabout 66.7 weight percent of present in an amount equal to about 50weight percent of the the said composition with the balance being sodiumhydroxsaid composition with the balance being potassium hydroxide. ide.5 m m w: 41 1:

2. The process of claim 1 wherein the temperature of the solution rangesfrom about 150* to about 210* F.
 3. The process of claim 1 wherein theconcentration of the said peroxydiphosphate compound is from about 5 toabout 120 grams per liter.
 4. The process of claim 1 wherein the saidperoxydiphosphate compound is potassium peroxydiphosphate.
 5. Theprocess of claim 1 wherein the concentration of the alkali metalhydroxide is from about 60 to about 120 grams per liter.
 6. The processof claim 1 wherein the alkali metal hydroxide is sodium hydroxide. 7.The process of claim 1 wherein the alkali metal hydroxide is potassiumhydroxide.
 8. The process of claim 1 wherein the said article isimmersed in the hot solution for about 3 to about 30 minutes.
 9. Theprocess of claim 1 wherein the said article is immersed for about 3 toabout 30 minutes in a solution comprising water, potassiumperoxydiphosphate and sodium hydroxide maintained at a temperature ofabout 150 to about 210* F.
 10. The process of claim 1 wherein the saidarticle is immersed for about 3 to about 30 minutes in a solutioncomprising water, potassium peroxydiphosphate and potassium hydroxidemaintained at a temperature of about 150* to about 210* F.
 11. Anaqueous solution for use in forming an adherent black coating on thesurface of a copper or copper alloy article comprising, in addition towater, a peroxydiphosphate compound selected from the group consistingof ammonium peroxydiphosphate, an alkali metal peroxydiphosphate and analkaline earth metal peroxydiphosphate, and mixtures thereof, and analkali metal hydroxide selected from the group consisting of sodiumhydroxide and potassium hydroxide and mixtures thereof, saidperoxydiphosphate being present in solution at a concentration of atleast 5 grams per liter and the ratio of peroxydiphosphate to hydroxidebeing at least 1:12 on a dry weight basis.
 12. The aqueous solution ofclaim 11 wherein the concentration of the peroxydiphosphate compound isfrom about 5 to about 120 grams per liter and the concentration of thealkali metal hydroxide is about 60 to about 120 grams per liter.
 13. Theaqueous solution of claim 12 wherein the said peroxydiphosphate compoundis potassium peroxydiphosphate.
 14. The aqueous solution of claim 11wherein the alkali metal hydroxide is sodium hydroxide.
 15. The aqueoussolution of claim 11 wherein the alkali metal hydroxide is potassiumhydroxide.
 16. A dry composition useful in forming an adherent blackcoating on the surface of a copper or copper alloy article comprising aperoxydiphosphate compound selected from the group consisting ofammonium peroxydiphosphate, an alkali metal peroxydiphosphate, and analkaline earth metal peroxydiphosphate, and mixtures thereOf, and analkali metal hydroxide selected from the group consisting of sodiumhydroxide and potassium hydroxide, said peroxydiphosphate being presentin a weight ratio of from 1: 12 to 2:1 relative to said alkali metalhydroxide.
 17. The dry composition of claim 16 wherein the saidperoxydiphoshpate composition is potassium peroxydiphosphate.
 18. Thecomposition of claim 16 wherein the said peroxydiphosphate compound ispotassium peroxydiphosphate present in an amount equal to about 66.7weight percent of the said composition with the balance being sodiumhydroxide.
 19. The composition of claim 16 wherein the saidperoxydiphosphate compound is potassium peroxydiphosphate present in anamount equal to about 50 weight percent of the said composition with thebalance being potassium hydroxide.